1. Field of the Invention
The invention relates to a vacuum belt conveyor.
2. Description of the Related Art
Vacuum belt conveyors are used to facilitate the threading of a paper web into a machine for the production, finishing or processing of a paper web. When a papermaking machine is started (or restarted after a web break) a narrow xe2x80x9ctailxe2x80x9d or lead-in strip is cut from the running web. This tail is transferred by means of the vacuum belt conveyor, e.g., from the end of a machine section to the infeed area of a following machine section.
In U.S. Pat. No. 3,355,349, a vacuum belt conveyor is disclosed having a vacuum box within the loop of the conveyor belt. The conveying run of the belt is traveling directly across the open surface (e.g., across a cover plate comprising suction openings) of the vacuum box, so that the negative pressure propagates through the conveying run of the belt in order to draw a web or a tail to be guided by the conveyor belt by suction. A side wall of the vacuum box is connected via a vacuum pipe to a vacuum source which is usually positioned at a certain distance from the vacuum belt conveyor. In many cases it is necessary to mount the vacuum belt conveyor pivotably to a stand or frame so that the conveyor can be moved to a non-functional position and back to a functional position where it is ready to perform a further threading operation. For that reason the vacuum pipe (which must be rather voluminous) must be designed as a flexible hose. This flexible hose is generally a disturbing factor.
In U.S. Pat. No. 4,692,215, a vacuum belt conveyor is disclosed which tries to avoid a vacuum box, an external vacuum source and a vacuum pipe therebetween. Arranged within the loop of the conveyor belt, in close proximity to the inner side of the conveying run, are so-called pneumatic guide plates or xe2x80x9cair traysxe2x80x9d. Air jets are directed over the guide plates in the direction of belt travel so that a negative pressure is created in order to draw a web or a tail to be guided on the conveyor belt by suction. Means are provided to adjust the overall pressure distribution in the traveling direction of the conveying run of the conveyor belt. However, there is a danger that the negative pressure fluctuates along the belt travel direction depending on the positioning of the guide plates. The resulting vacuum pulsation can cause problems in the threading operation, especially if the conveyor is inverted. In this case the web or tail may fall off the conveyor, especially if an overpressure is produced to detach the web or tail from the downstream end of the conveying run of the belt as disclosed in FIGS. 5A and 5B of US ""215. Even in the normal position of the conveyor (conveying belt run on top side), there is a danger that the pneumatic guide plates produce a too high over-pressure at the downstream end of the conveyor so that the web or tail jumps off in an uncontrollable manner. Possibly, this disadvantage results from the fact that the guide plates do not have positive means for the discharge of the air.
The known conveyor (US ""215) also requires that the pneumatic guide plates be put in contact with the inner side of the belt in order to get the required level of vacuum. This makes it more prone to plugging and other problems associated with paper being sucked into the conveyor.
The present invention provides an improved vacuum belt conveyor which avoids a flexible pipe connection from the belt conveyor to an external vacuum source while, nevertheless, the vacuum belt conveyor, if needed, remains movable between various positions.
The invention at least minimizes the length of a pipe connection from a vacuum belt conveyor to its vacuum source or avoids the pipe connection completely.
The present invention provides an improved vacuum belt conveyor wherein the level of the negative pressure is as stable (or continuous) as possible along the travel path of the conveying run of the belt, in order to obtain an optimized threading operation.
The invention provides the option that any over-pressure or backpressure is avoided at the downstream end of the conveyor.
Also, the invention provides an improved belt conveyor which avoids plugging or sucking paper into the interior of the conveyor.
The aforementioned features as well as further features that will be described herinafter are attainedxe2x80x94according to a first embodiment of the inventionxe2x80x94by a novel position of a vacuum blower, namely within the loop of the air-pervious endless belt. The vacuum blower can be positioned within the loop of the belt in such a way that the inflow of the blower is arranged in close proximity to the inside of the conveying run of the belt. In this case, the at least one vacuum blower is arranged in the place of the formerly provided vacuum box. However, in a second embodiment, the vacuum belt conveyor includes a vacuum box, and the at least one vacuum blower is positioned inside the vacuum box near the return run of the belt, with the inflow of the blower being open towards the open surface (e.g., to a cover plate having suction openings) of the vacuum box. There may be provided a certain distance between the blower""s inflow and the open surface.
The second embodiment also allows retrofiting an existing vacuum belt conveyor by installing a vacuum blower inside a conventional vacuum box.
It should be understood that a vacuum blower of very compact design can be used, the space inside the belt loop being relatively small. The vacuum blower is driven by an air turbine. The benefit of such an air turbine is that it has a very small overall height (measured along the rotational axis). This is an important advantage for the positioning of the vacuum blower within the loop of the belt.
If needed, the vacuum blower (being positioned inside the belt loop) may be driven by an electric motor, which again can have a very small overall height. The vacuum belt conveyor can be supported by a rotatable or pivotable support, so that the complete vacuum belt conveyor including the vacuum blower can be moved, e.g., from a functional position to a non-functional position and back to the functional position.
According to the invention, at least one air turbine driven vacuum blower developed by MISCEL OY, LTD., Tampere-Finland can be used.
According to a second embodiment of the invention, the features mentioned above are attained by creating the negative pressure (required in the belt loop) by use of an air stream induced by a propulsion jet, with the air stream and the propulsion jet being joined to a xe2x80x9ccombined flowxe2x80x9d which is positively directed to the outside of the belt conveyor.
The various embodiments of the invention have in common that the following further advantages are attained;
The necessity of providing an external voluminous vacuum pipe is eliminated, because the vacuum source (vacuum blower or propulsion jet device) is positioned inside of the conveyor, namely inside of the belt loop. However, at the same time, the discharge of the air emitted by the vacuum blower (and by the air turbine if existing) or by a propulsion jet device can be reliably controlled, so that the emitted air does not disturb the travel of the paper web or threading tail. Also, the airflow produced by the blower and/or the air turbine or produced by the propulsion jet keeps the vacuum belt conveyor much cleaner than with previous known conveyor designs, in particular if the discharge air is directed through the return run of the belt.
By use of the invention, further advantages are obtained, namely improved accessibility to the vacuum components, e.g., for maintenance and service. Also, a constant vacuum level along the conveying run of the belt is achieved. Furthermore, at the downstream end of the conveying run of the belt, the web or tail can be detached from the belt by an airjet which flows through the air-pervious belt in a conventional manner or, even more reliably, by a nose shoe designed according to U.S. Pat. No. 4,022,366. That nose shoe avoids the need of any backpressure in the interior of the belt loop.